1. The Field of the Invention
The present invention relates to a method for evaluating an initial capacity of secondary batteries and, more specifically, to a method for evaluating an initial capacity of secondary batteries that involves mathematical operation of specific internal resistance components obtained from an equivalent circuit model of an impedance spectrum measured for a short time period instead of using direct discharge characteristics, thereby allowing selection of batteries having a different initial discharge characteristic.
2. Related Prior Art
As the electronic appliances, office supplies and mobile communication equipment are increasingly required to be more light-weigh and portable in recent years, it is necessary to manufacture secondary batteries used as a driving power source for those products in a miniaturized, light-weight and high-energy compacted way.
Among the secondary batteries, the lithium-ion secondary batteries that are designed to have a high battery voltage of 3 to 4 V and a high energy density of 100 Wh/kg and reversible in insertion and release have been developed to have a battery capacity increased from 0.6 Ah level to 2 Ah level. The lithium-ion secondary batteries are commercially available in various forms, rectangular, cylindrical or the like, according to the demand on the market. These high-capacity batteries are in an increasing demand and expected to be a principal energy source for transmission of picture information in the future, as the recent mobile communication terminals are turned into multimedia devices. Typically, the high-capacity batteries are used in the form of a single-unit battery (for appliances having a single battery as a power source) or a multi-unit battery group (i.e., a battery pack including multiple battery units in serial, parallel, or serial-parallel arrangement). In the case of the multi-unit battery group, the evaluation of capacity for batteries is of a great significance. It is known that the battery manufacturers evaluate the capacity of batteries by comparing impedance measurements at a specific frequency and a charge/discharge test repeatedly performed with a predetermined frequency.
Conventionally, the methods for measuring the capacity of batteries involve measuring the impedance of the batteries at a specific frequency or examining the state of charge/discharge of the batteries.
U.S. Pat. No. 3,562,634 discloses a method for determining the state of charge of Ni—Cd secondary batteries from the Faraday capacity measured using a bridge. According to this reference document, the correlation between the internal impedance of the battery at a specific frequency and the battery capacity is dependent on the impedance response characteristic of the chemical material used as an active material of the battery. So this method is not applicable to all kinds of batteries for capacity measurement.
Another method of measuring the internal impedance of batteries that is more generally used is disclosed in U.S. Pat. No. 4,678,998, in which the state of the batteries can be determined from the correlation between the internal impedance value at a specific frequency and the battery capacity. This method monitors the state of the batteries in a vehicle in a continuous manner and comparing impedance values measured at the respective frequencies.
Besides, U.S. Pat. No. 4,743,855 discloses a method using two complex impedance values measured in low and high frequency regions, respectively, and U.S. Pat. No. 5,241,275 discloses a method using a linear impedance characteristic in the low frequency region. Also, a method using the correlation between the impedance value at a specific frequency or in a specific frequency region and the capacity of the battery hardly guarantees the efficiency of measurement and the precision of the correlation.
The latter method has a problem in the aspect of time and economy in evaluating the capacity of numerous batteries using charge/discharge equipment.
The general methods for determining the state of charge/discharge of batteries involve measurements of the open circuit voltage, the battery voltage and variations in the operation mode, the characteristic of output signals for an input voltage or current applied to the batteries and an internal resistance function or an impedance function derived from the characteristics. The use of these methods allows a measurement of the battery capacity in a shorter time period than a required time for a real-time discharge method. For this purpose, many studies have been made. For example, U.S. Pat. No. 4,952,862 discloses a method for calculating a residual capacity of batteries that involves expressing the discharge characteristic of the batteries as a voltage-time function consisting of Peukert parameters and calculating the residual capacity of the batteries from the voltage measurement of the batteries and the voltage-time function. EP No. 119,547 also suggests a method for measuring the discharge voltage of batteries during a discharge as a function of time and then determining the state of discharge of the batteries from the average variation of the discharge voltage in a predefined interval. U.S. Pat. No. 3,984,762 discloses a method for measuring a phase difference between an AC applied to a battery and a measured AC voltage. In the case of calculating the capacity by measuring a voltage or a voltage change in the course of a discharge, the precision of the correlation between the measured value and the capacity is dependent on the discharge characteristic of the battery. For example, this method cannot be applied to lithium-ion secondary batteries, which exhibit uniformity of voltage and manufactured are in the same conditions, because the voltage change of the batteries is so insignificant with respect to the change of the state of discharge.